The present invention relates to an electroless gold plating bath and electroless gold plating method used when gold plating coating is to be formed on parts used in the electronic industry, such as a printed wiring board or indium-tin-oxide (xe2x80x9cITOxe2x80x9d) substrate. In particular, the present invention relates to an electroless gold plating bath and method whereby base metal etching or erosion arising when gold is deposited on the material to be plated is extremely slight, so that a gold film with good adhesion, and good soldering strength is obtained.
Conventionally, gold plating has been utilized on the surfaces of electronic parts such as printed circuit boards, ceramic integrated circuit (xe2x80x9cICxe2x80x9d) packages, ITO substrates and IC cards in order to improve chemical resistance, oxidation resistance and physical properties such as metal conductivity, soldering properties, thermo-compression bonding properties and other connection properties. With most of these parts in the electronics industry, it is necessary to carry out gold plating in electrically isolated regions, and so the use of electrolytic gold plating is inappropriate. Electroless gold plating must be performed.
Conventional technologies that are widely known are the use of autocatalytic electroless gold plating baths whereby gold is deposited due to the action of a reducing agent having catalytic activity with respect to gold, and substitution (displacement) plating baths whereby gold is deposited along with dissolution of base metal such as nickel. These two technologies are widely used at present, and represent typical electroless gold plating baths.
With substitution gold plating, gold is deposited by replacement of the base metal, and so dissolution of base metal (etching or erosion) occurs along with deposition of the gold. With conventional substitution gold plating baths, the substitution reaction rate is not controlled, and so the substitution reaction rate is particularly rapid immediately after initiation of the reaction. A large number of defects are formed in substitution gold films due to this rapid rate immediately after initiation of the reaction, and thus these defect regions connect and accumulate, so that the base metal present under the gold film is excessively etched or corroded in the depthwise or transverse direction. When this type of substitution gold plating bath is used for gold plating, sites where the structure of the crystal grain boundaries or other structures of the base metal are weak are preferentially dissolved (etched or eroded).
It is thought that etching or erosion in the form of deep crevices thus occurs along grain boundaries in base metals after formation of gold films when a conventional substitution gold plating bath is used.
For example, when a substitution gold deposit with at thickness of 0.05-0.1 xcexcm (micron) has been formed on an electrolessly plated nickel film at a thickness of 5 xcexcm under common electroless nickel-gold plating specifications using known electroless nickel plating and substitution gold plating baths, the cross-section of the coating is found, by use of a scanning electron microscope, to have grooves formed under the gold film due to progressive deepening of erosion in grain boundary regions of the deposited grains. This erosion results from selective strong attack on the grain boundary regions of the deposited particles of the electroless nickel film by the gold plating solution. Although the film thickness of the deposited gold is thin, at 0.1 xcexcm or less, the erosion depth is as much as 3-5 xcexcm. As a result, the electroless nickel film that is formed by this type of substitution gold plating is rendered brittle, and has inferior adhesion to the gold film. In particular, the material will not withstand soldering, and thus has poor practical utility.
On the other hand, when autocatalytic electroless gold plating baths are used, immediately after immersion of the material to be plated in the plating bath, gold is deposited due to the substitution reaction occurring between the base metal and gold, and subsequently, the action of the reducing agent is initiated with the deposited gold as catalyst. Due to this two-stage reaction whereby gold is deposited, it is not possible to completely prevent etching or erosion of the base metal by the gold plating bath.
Plated films of this type have insufficient adhesion and tend to peel during durability testing. Sufficient solder strength cannot be ensured when soldering is carried out, and thus the materials tend to have poor soldering properties in solder strength tests due to exposure of the base metal.
In addition, microprocessor packages have continued to proliferate in recent years, and with ball-grid array semiconductor packages that are manufactured using printed wiring board technologies, it is necessary to perform gold plating with the objective of improving solder adhesion properties on electrically isolated patterns. However, conventional electroless gold plating technologies have serious problems in terms of the generation of defective products due to insufficient solder adhesion strength. For this reason, gold plating is currently carried out by electrolytic plating methods when improved solder adhesion properties are desired.
The present invention has the objective of offering an electroless gold plating bath whereby a plated gold layer with improved adhesion with respect to base metal can be formed without erosion of the base metal.
In addition, the present invention has the objective of offering an electroless gold plating method whereby a plated gold layer with improved adhesion with respect to base metal can be formed.
The inventors of the present invention et al., carried out painstaking investigations with the objective of attaining the above objectives, and arrived at the present invention upon discovering that the above objectives can be attained by means of using an electroless gold plating bath that contains a combination of specific components.
Specifically, the present invention concerns an electroless gold plating bath for depositing a gold film on a material to be plated having metal at its surface, where said electroless gold plating bath comprises, (i) a water-soluble gold compound, (ii) a complexing agent that stabilizes metal ions in the plating bath, but does not allow substantial dissolution of nickel, cobalt or palladium in the plating bath, and (iii) a polyethyleneimine compound. Also provided by the present invention is an electroless gold plating method that employs the aforementioned electroless gold plating bath. In this method, a metal on the surface of a material to be plated with gold is contacted with the above described plating bath, for a period of time sufficient to deposit a gold layer in a desired thickness. Specifically, the present invention is an electroless gold plating bath for producing electroless gold plating on a material to be plated having metal at its surface.